Definitions

The rationalist community is a group of people (of which I’m a part) who met reading the site Less Wrong and who tend to hang out together online, sometimes hang out together in real life, and tend to befriend each other, work with each other, date each other, and generally move in the same social circles. Some people[7] call it a cult, but that’s more a sign of some people having lost vocabulary for anything between “totally atomized individuals” and “outright cult” than any particular cultishness.

But people keep asking me what exactly the rationalist community is. Like, what is the thing they believe that makes them rationalists? It can’t just be about being rational, because loads of people are interested in that and most of them aren’t part of the community. And it can’t just be about transhumanism because there are a lot of transhumanists who aren’t rationalists, and lots of rationalists who aren’t transhumanists. And it can’t just be about Bayesianism, because pretty much everyone, rationalist or otherwise, agrees that is a kind of statistics that is useful for some things but not others. So what, exactly, is it?

This question has always bothered me, but now after thinking about it a lot I finally have a clear answer: rationalism is the belief that Eliezer Yudkowsky is the rightful caliph.

No! Sorry! I think “the rationalist community” is a tribe much like the Sunni or Shia that started off with some pre-existing differences, found a rallying flag, and then developed a culture.

...typical rationalist philosophical positions include reductionism, materialism, moral non-realism, utilitarianism, anti-deathism and transhumanism. Rationalists across all three groups tend to have high opinions of the Sequences and Slate Star Codex and cite both in arguments; rationalist discourse norms were shaped by How To Actually Change Your Mind and 37 Ways Words Can Be Wrong, among others.

...a community that call themselves Rationalists, that read ‘high-IQ sites’ such as Marginal Revolution, Less Wrong, and Slate Star Codex, and according to various surveys, identify as liberal, are atheist or agnostic, and, in general, hold a ‘realist’ philosophical worldview.

Rationalist-adjacent

There are people who agree on few to no rationalist positions but still like going to our parties and reading our blog posts. I coined the term “rationalist-adjacent” for this group before I got the idea that the names of all subdivisions of the rationalist community should begin with the letter C...
A lot of Less Wrong references a lot of nerd culture, such as catgirls, anime, fanfiction, Harry Potter, My Little Pony, etc